The present invention relates to an internal combustion engine having a system of coolant passages formed in or around a cylinder block and a cylinder head.
One example of the coolant passage system is shown in Japanese Utility Model examined publication No. 13443/1979. In this example, there is formed, in an upper portion of a cylinder block, a coolant distributor passage extending along a row of cylinders. The coolant distributor passage serves to supply the cooling water to the vicinities of all the cylinders simultaneously so that all the cylinders can be cooled equally. However, the coolant collector passage is not sufficient to cool the row of cylinders uniformly because there is a tendency for the cooling water to flow longitudinally along the cylinder row in the water jacket of the cylinder head.